The gear knob just needs brute force to remove. Grab hold of it and pull it upwards hard. A bit of twisting will help. There is a spring under the knob which will likely disappear into the distance.
Genuine Replacements are available for about £40 from Partsworld interweb store.
Aftermarket knobs don't fit well because the collar for the reverse lift is huge, and the shaft is splined. Grub screw fixings just don't fit well at all.
To fit a new OE one, you fit it on the shaft and whack it really hard with a rubber mallet. Cover the gear knob with a leather shammy or similar to protect it. Again, pure brute force.
I used a wooden mallet, a bit of off cut softwood and a leather shammy to fit mine. I put a cross cross pattern on the first replacement I fitted by using a microfibre cloth to 'protect' it. The microfibre pattern etched itself into the aluminium top.